The Mirador 1000 is a massive, white condominium building fronted by palm trees in Miami's South Beach neighborhood. It's got a pool that looks directly out onto the ocean, a beautiful lobby and its own workout room. It's within walking distance of South Beach's best restaurants, shopping and clubs. A unit in the building is, in other words, a prime find on Airbnb if you're booking a weekend trip to Miami.

The problem though is that short-term rentals aren't allowed, both due to condo rules and because the building is in an illegal zone for such rentals, says Arianna Aguero, president of the Mirador 1000's condominium association. Aguero says that the 400-plus-unit building doesn't like short-term renters through Airbnb, VRBO, or HomeAway because you have people coming into the building "with no information about who they are," as opposed to long-term renters who have filled out applications and had background checks. Plus, the people who vacation in South Beach for the weekend are the type of party people likely to disrupt the community of year-round residents.

So the Mirador 1000 has for the last few years played a cat-and-mouse game with the people in the building Airbnb'ing their units, trying to figure out which guests coming into the building are paying a per-night fee to be there.

"We didn't have a good solution except for our front desk people trying to find out," said Aguero by phone. It's easiest when someone arrives and tries to "check in" at the front desk. If building management realizes a "guest" is actually a paying customer, it turns the person away, "reservations" be damned. "But people who want to do it find ways around it, saying the guests are friends or meeting elsewhere and walking them in," said Aguero.

In February, the building turned to a new tool called STRMonitor, that, for $77 per month, monitors a number of short-term-rental sites and lets a building know if any of their tenants' apartments appear there.

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"At least 15 people have been caught in my building," said Aguero. "We send a letter to the owner of the unit and even do a fine. Sometimes the owner isn't aware that the renter is doing a short term rental."

Airbnb is fighting battles across the country with city governments around the regulations that should exist around people turning their homes into temporary hotels. But on smaller, more personal battlefields, renters are duking it out with landlords and building managers over the appropriateness of their profiting from re-leasing all or part of their units. Lots of leases forbid subletting so many people put their rented apartments on Airbnb on the sly, even though Airbnb advises against this. If they're busted, it means a guest might get kicked out mid-stay or be prevented from staying at all, and it could mean the Airbnb host is suddenly homeless as well. Airbnb's comment forums are littered with tales of people getting evicted for renting out their units without a landlord's knowledge.

Nicolas Lund-Larsen, the CEO of Miami-based STRMonitor, is a real estate broker and home owner's association president who helped launch the company a year ago. The company is now working with 30 buildings. Mostly its customers are high-rises in Florida, though it's a global service.

From the company's brochure

"A lot of managers and condo association boards had an administrative assistant spending hours going through all the different sites looking for units," said Lund-Larsen by phone. "Our service automates this."

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Lund-Larsen set me up with a temporary STRMonitor account, and each week, it sent me emails letting me know when units from a property in Florida appeared on various sites. The service, though, just tells you that a unit in your building is listed on a site. If you can't tell from the person's name or the photos which unit it is, you can pay extra to have STRMonitor's private investigator figure out who the unit belongs to, usually based on details mentioned in the reviews.

"If you have a building with 1,200 units, you probably need help," said Lund-Larsen. "We're really good at solving cases."

Lund-Larsen says that most tenants take down the listing once they've been busted. The buildings that use his service have issued fines and turned off the tenants' fobs so they can't access the building until they talk to management.

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"I don't mind Airbnb," said Lund-Larsen. "I think it's a great business but I mind that people are renting out in a building where it's not allowed. That's not a sharing economy, it's a taking economy."